1. Technical Field of the Invention
The present invention relates generally to a spread spectrum receiver for use in communication system, and more particularly to an improved circuit structure of a spread spectrum receiver for cellular telephone systems which is designed to select input signals suitable for diversity combining out of multipath signals traveling on different transmission paths.
2 Background of Related Art
In recent years, there is an increasing need for terrestrial mobile communications such as cellular telephone systems, and techniques of improving spectrum efficiency are becoming important to increase a user capacity within a limited frequency band. As one of multiple access techniques, code division multiple access (CDMA) is proposed. CDMA can realize high-quality communication by means of spread spectrum communication techniques using sharp correlation properties with respect to a pseudo noise sequence within a wide band. The use of CDMA techniques in terrestrial mobile communication systems is disclosed in U.S. Pat. No. 4,901,307 to Gilhousen et al., issued on Apr. 28, 1992, disclosure of which is incorporated herein by reference. In spread spectrum communications using a direct sequence system, multi-path components are subjected to Maximal-Ratio Combining in a receiver called a rake receiver in order to enhance the diversity effects. The rake receiver is disclosed in, for example, U.S. Pat. No. 5,109,390, disclosure of which is incorporated therein by reference.
FIG. 7 shows a conventional spread spectrum communication system using the direct sequence.
A transmit data signal 49 is inputted to an information modulator 50. The information modulator 50 outputs a narrowband signal having a bandwidth required only for transmitting the data signal 49. The bandwidth of an output of a spreading code generator 51 is much wider than that of the output of the information modulator 50. The spread spectrum modulator 52 multiplies the output from the information modulator 50 by a spreading signal such as a pseudo noise sequence outputted from the spreading code generator 51 to spread the narrowband signal over a broader bandwidth and outputs it through the transmitter antenna 53. In a receiver, the spread spectrum demodulator 56 changes a wideband signal received by the receiver antenna 54 into a narrowband signal. This conversion is accomplished by multiplying the received wideband signal by a spreading code outputted from a spreading code acquisition circuit 55 which is the same as that outputted by the spreading code generator 51.
Usually, an interference signal caused by a signal outputted from another transmitter or thermal noise is, as shown in FIG. 7, added to the transmit signal in the course of transmission. The use of the spreading code provided by the spreading code acquisition circuit 55 having a much smaller cross-correlation to the interference signal, however, results in a decrease in interference component of the output from the spread spectrum demodulator 56.
In general, a mobile communication network is, as shown in FIG. 8(a), subject to multipath. The mobile station 60 receives a direct wave transmitted directly from the base station 59 through the transmission path 62 and also receives a delayed wave reflected from the building 61 through the transmission path 63. FIG. 8(b) shows power levels of the direct wave and the delayed wave. In the receiver, demodulation of a signal transmitted from particular one of the paths is achieved by matching the timing with which the spread spectrum demodulator 56 operates, that is, the phase of the in spreading code produced by the spreading code acquisition circuit 55 to that of the signal from that path. In such multipath communications, a delayed wave interferes with a direct wave.
The so-called rake receiver includes a plurality of spread spectrum demodulators which operate with different timings to perform path diversity combining of received signals.
FIG. 9 shows a spread spectrum demodulator circuit of the rake receiver. FIG. 10 shows power levels of signals arriving at the receiver at different times t.sub.0, t.sub.1, t.sub.2, t.sub.3, and t.sub.4 from different directions.
The path level detector 12 determines power levels of received waves 1 (i.e., multipath signals 16 to 20) and phases thereof. The phase assignment circuit 14 determines phases of modulating operations (i.e., signal reception timings) of the spread spectrum demodulators 2 to 5, respectively, based on the output 13 from the path level detector 12. This phase determination is made so as to allow the rake receiver to perform maximal-ratio combining of the received signals 1.
A mobile communication network may experience Rayleigh fading so that the power level of each multipath signal drops momentarily. Each faded multipath signal changes in power level over 20 dB. The rake receiver minimizes deterioration in reception quality by combining such faded multipath signals, which is commonly called Path Diversity.
The above conventional spread spectrum demodulator circuit, however, has the drawback in that changes in power level of the multipath signals due to Rayleigh fading may preclude maximal-ratio combining in the diversity combiner 10, resulting in deterioration in signal reception quality.
An example of such a problem will be discussed below with reference to FIGS. 11 and 12. For the brevity of explanation, it is assumed that three multipath signals A, B, and C have arrived at the receiver in FIG. 9, and the receiver has only two spread spectrum demodulators 2 and 3.
FIG. 11 shows variations in power level of the multipath signals A, B, and C. FIG. 12 shows variations in power level of combined signals. Numeral 69 indicates the power level of the signal into which the multipath signals A and B are combined. Numeral 70 indicates the power level of the signal into which the multipath signals B and C are combined. Numeral 71 indicates the power level of the signal into which the multipath signals C and A are combined.
It is known that typical mobile communications may experience fading so that the amplitude of received signals change irregularly according to a Rayleigh distribution, but this change will be expressed here in a sine signal for the simplicity of explanation.
Average values of the power level 66, 67, and 68 of the multipath signals A, B, and C are, as shown in table 1 below, 1.7, 1.6, and 1.5, respectively. Therefore, if the phase assignment to the spread spectrum demodulator 2 and 3 is made based on the magnitudes of the averaged values of the power levels of the multipath signals 66 to 68, then components of the multipath signals A and B are combined in the diversity combiner 10. The combination of the multipath signals A and B is, however, unsuitable from the point of view of a drop in power level after diversity combining. The signal into which the multipath signals A and B are combined, as indicated at 69 in FIG. 12, drops in power level near time 4 and 10. This level drop will result in fatal deterioration in communication quality. The combination of the multipath signals A and C results in a smaller drop in power level as compared with the combination of the multipath signals A and B.
Such a drop in signal level after diversity combining is caused by a high correlation between multipath signals to be combined.
The correlation between multipath signals usually depends upon conditions of transmission paths. Particularly, in the case of mobile communications, the correlation changes momentarily and is impossible to estimate. It is known that when a difference in phase between multipath signals is small, the correlation is usually high, while when the difference in phase is great, the correlation is low, but the relation between the phase difference and the correlation depends upon a propagation environment and is difficult to estimate.
TABLE 1 ______________________________________ multipath A multipath B multipath C ______________________________________ average 1.70 1.60 1.50 square of aver. 2.89 2.56 2.25 means square 3.39 3.07 2.75 auto-cross 0.50 0.50 0.50 correlation ______________________________________